Big Data

Solving the Data Skew Headache in Spark

Skewed data — where one partition is 10x larger than others — is the number one reason Spark jobs fail. This lesson covers Salting techniques to redistribute skewed join keys across all executors.

Level: AdvancedBig Data

What You'll Learn

What data skew is and how to detect it
Diagnosing skew with the Spark UI and stage metrics
Salting: adding random prefixes to redistribute keys
Broadcast joins as an alternative to salting
AQE (Adaptive Query Execution) in Spark 3+
Implementing salting in PySpark step by step
Benchmarking job time before and after salting

What's Included

🧭AI Tutor🔒

Interactive learning path with AI-generated lessons read aloud by your personal tutor

🧪Practice Lab🔒

Step-by-step lab instructions with AI guidance — run in your own IDE or sandbox.

🎙️Mock Interviews🔒

AI interviewer asks tailored questions, gives real-time feedback and a score report

📝Evaluation🔒

MCQ questions (theory, practical, code) to test your knowledge and earn a badge

⚔️Challenge AI🔒

Compete head-to-head against AI on real production problems — judged on correctness, performance, scalability, code quality, and edge cases

Course Syllabus

Module 1: Understanding Data Skew

  • •What data skew is and why it stalls Spark jobs
  • •Identifying skewed partitions in the Spark UI
  • •Common causes: hot keys, NULL joins, categorical imbalance
  • •Measuring skew: partition size distribution
  • •When skew matters vs when it can be ignored

Module 2: Salting Technique

  • •Salting concept: appending a random prefix to join keys
  • •Step 1: add salt column to the large table
  • •Step 2: explode salt column on the small table
  • •Joining on the salted key and removing the prefix
  • •Trade-offs: storage overhead vs execution speed

Module 3: Advanced Skew Fixes

  • •Broadcast join: when the small table fits in memory
  • •AQE skew join optimisation in Spark 3+
  • •Skew hints: SKEW hint in Spark SQL
  • •Repartition vs coalesce for pre-join reshuffling
  • •Capstone: fix a skewed join and compare stage execution times

You're all set

Everything in this course is unlocked.

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✓AI Tutor
✓Practice Lab
✓Mock Interviews
✓Evaluation
✓Challenge AI